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Origins of Rocks:

Origins of Rocks:. Where do rocks come from? Why do they form? How do they form? (Mineralology and Petrology) (L2 & L3). What is a rock ?. A naturally-occurring aggregate of minerals CD: EM/A,B. What is a Mineral ?.

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Origins of Rocks:

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  1. Origins of Rocks: Where do rocks come from? Why do they form? How do they form? (Mineralology and Petrology) (L2 & L3)

  2. What is a rock? • A naturally-occurring aggregate of minerals • CD: EM/A,B

  3. What is a Mineral? • A naturally occurring, inorganic, crystalline solid with a specific chemical composition • A specific chemical composition means that the composition of a mineral can be expressed as a chemical formula • Example: halite (salt) is NaCl

  4. Identify Minerals by Their Physical Properties [See CD: Earth Materials] Specific Gravity Color Streak Luster Acid Test Crystal Habit Cleavage & Fracture * Striations Hardness

  5. What is a Crystal? • A form of matter which has a regular, repeating framework of atoms

  6. Halite (salt) crystals (NaCl)

  7. What are atoms?[CD:Slides 1094-1105 in Geo Time] • The smallest unit of an element that retains the element’s physical and chemical properties • An element is a substance that cannot be broken down into a simpler substance • Made up of protons, neutrons and electrons • examples: gold, iron, hydrogen, oxygen, sodium, chlorine, carbon, silicon, helium

  8. Protons have positive charge & mass of 1 Neutrons have no charge & mass of 1 Protons and neutrons make up the nucleus of an atom Electrons have a negative charge, NO mass and are most important for chemical reactions Protons, Neutrons, Electrons

  9. Structure of an Atom Protons +/1 Neutrons 0/1 Electrons -/0

  10. IONS +/- CHARGED ATOMS FORMERLY NEUTRAL ATOMS WHICH HAVE GIVEN UP (+) OR TAKEN ON AN ELECTRON (-) INVOLVED IN IONIC BONDING ISOTOPES ELEMENTS WITH VARIABLE ATOMIC WEIGHT DUE TO VARYING NUMBERS OF NEUTRONS IN THE NUCLEUS. IONS & ISOTOPES

  11. Hydrogen atom: Mass = 2 1 proton, 1 electron Carbon atom: Mass = 12 6 protons, 6 neutrons, 6 electrons Electrons move in orbitals (not “orbits!”) forming shell-like spheres around the nucleus. ** If atoms loose or gain electrons they have an electrical charge (-/+) and are called ions; also, a group of atoms (molecule) can behave as an ion if they have too many or too few electrons.

  12. What holds the atoms togetherin crystals? • Chemical bonds: • Ionic Bonds • atoms (ions) held together by their electricalcharges • weaker kind of chemical bond • Covalent Bonds • atoms held together because they share electrons • stronger kind of chemical bond

  13. Example of an ionic bond: NaCl

  14. Example of covalent bonding: Carbon Naturally occurring crystalline carbon is the mineral diamond Covalent bonding between the carbon atoms make this the hardest known substance

  15. WHAT FACTORS DETERMINE WHAT MINERAL WILLFORM? 1 WHAT ELEMENTS ARE AVAILABLE 2 ABUNDANCE OF EACH ELEMENT 3 SIZE & CHARGE OF EACH ELEMENT 4 ENVIRONMENT - SPACE, PRESSURE, TEMPERATURE

  16. Ionic Radii & charge of some geologically important ions allows “ionicsubstitution” or SOLID SOLUTION

  17. Average Composition of the Continental Crust O O Si Weight Percent Volume Percent

  18. COMMON ROCK-FORMINGMINERAL FAMILIES NATIVE ELEMENTS - GOLD, COPPER HALIDES - PRECIPITATES (salts) OXIDES - 1 METAL + OXYGEN SULFIDES - 1 METAL + SULFUR SULFATES - (SO4)2- CARBONATES - (CO3)2- SILICATES - SUPER STARS!!!

  19. Silicates • The largest group of minerals are the silicates • Silicates are distinguished by being composed of the silicate ion: (SiO4)4- • an ion of Silicon has a charge of 4+ • four ions of Oxygen have a charge of 8-

  20. Silicate ions have a tetrahedral shape • In other words, they’re four-sided pyramids

  21. Tetrahedrons can link together to make complex crystals isolated ring single chain double chain sheet framework--like sheet but in three dimensions

  22. Where do rocks come from? • All rocks are made from other rocks! • Three basic families of rocks based on their origin: • Igneous • Sedimentary • Metamorphic • CD: EM/C.

  23. Igneous Rocks • Igneous rocks formed when their constituent minerals crystallized out of molten rock as it cooled from a high temperature • Three ways to melt rocks: • increase in temperature - hot spots • decrease in pressure - mid-ocean ridges • add water - subduction zones

  24. Where do Igneous rocks form? • Inside the earth (intrusive/Plutonic) • in cracks: dikes • in-between rock layers: sills, laccoliths • in magma chambers • magma is molten rock beneath the surface of the Earth • Surface of the earth (extrusive/Volcanic): • lava flows • pyroclastics: pumice and ash

  25. Sedimentary Rocks? • Formed from sediments settling out of a fluid • sediments are: • particles worn off of other rocks • tiny mineral grains precipitated out of a fluid • for example, salt, gypsum, limestone • created by biologic activity • Fluid can be either air or water • mudstones form in water • petrified sand dunes form in air

  26. Metamorphic rocks? • Formed when rocks are exposed for a long time to a different (usually higher) pressure / temperature than which they formed in • most minerals are only stable at a particular pressure and temperature range • Examples: • high pressure: downgoing slab in subduction zones • high temperature: rocks next to magma/lava • high pressure and temperature: roots of mountain ranges

  27. high pressure high temperature

  28. The Rock CycleCD: Earth Materials, C. • Since every rock is formed out of pre-existing rocks, the evolution of rocks can be shown to be a rock cycle, which illustrates how every rock type can be formed out of the other two types.

  29. Igneous RocksCD:EM/D • There are a wide variety of igneous rocks, and they can be better understood if we sort them by their: • Texture • size distribution of mineral grains • Chemical composition • types of minerals present

  30. Igneous Rock Textures • Aphanitic (Fine) Texture • Very tiny crystals • Indicates rapid cooling: extrusive or volcanic rocks • volcanic rocks form on the surface or shallow subsurface • Phaneritic (Coarse) Texture • Large crystals • Indicated slow cooling: intrusive or plutonic rocks • plutonic rocks form underground • Porphyritic (Mixed) Texture • Large crystals set in a matrix of tiny crystals • Indicates slow then fast cooling

  31. Aphanitic Texture (Fine)

  32. Phaneritic Texture (Large)

  33. Porphyritic Texture (Mixed)

  34. Igneous Rock Chemistry • Most igneous rocks are made of: • Oxygen (O) • Silicon (Si) • Aluminum (Al) • Iron (Fe) • Magnesium (Mg) • Calcium (Ca) • Sodium (Na) • Potassium (K)

  35. Igneous Rock Chemistry • All igneous rocks can be described as mafic, intermediate or felsic • Mafic Igneous Rocks • Dominated by dark-colored mineral grains • High Fe, Mg, Ca • Low Si • Felsic Igneous Rocks • Dominated by light-colored mineral grains • Low Fe, Mg • High Si

  36. Igneous Rock Chemistry • Finally, igneous rocks can be further subdivided on the basis of how much Sodium (Na) and Potassium (K) they contain • If they have more Na and K than Ca, they are considered to be alkaline igneous rocks (Felsic)

  37. Chemistry and Minerals • The kind of minerals an igneous rock contains results from the chemical composition of the parent magma, the molten rock from which the rock formed • The chemistry of the parent magma is the result of two processes: • Partial melting of a source rock • Fractional crystallization of the magma as it cools

  38. Partial Melting • Any given rock is made of several different minerals with different melting temperatures • When a rock begins to melt, some minerals will melt before others • This process results in a melt which has a different chemical composition than the rock from which it is forming

  39. Fractional Crystallization • As a magma cools, crystals will start to grow within it • These crystals remove elements from the magma, changing the chemical composition of the remaining melt • This remaining melt can then grow crystals different from those that have already grown from it

  40. Bowen’s Reaction Series

  41. Classification of Igneous Rocks volcanic felsic intermediate mafic ultramafic plutonic

  42. Hawaiian Igneous Rocks • It takes a great many steps of fractional crystallization to make a felsic rock out of a mafic one • In Hawaii, most of the magma was erupted too quickly for it to be anything other than a mafic, aphanitic rock: a basalt • Therefore, the most common minerals found in Hawaiian igneous rocks are olivine and Ca-rich plagioclase

  43. Another way of looking at it...

  44. Magma & Plate Tectonics • Ultramafic magmas - mantle source • Mafic magmas - partial melt of mantle • Intermediate magmas - “ “ of oceanic crust • Felsic magma - partial melt of continental crust • Don’t forget that the “mother of all rock” is PERIDOTITE.

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